Mineralogy and Geochemical Appraisal of Paleo-Redox Indicators in Maastrichtian Outcrop Shales of Mamu Formation, Anambra Basin, Nigeria

Abstract

The Mamu Formation exhibits two types of shales, viz. grey and dark shales. The geochemical and mineralogical compositions of these shales were investigated using X-ray fluorescence (XRF) and Laser Ablation- Inductively Coupled Plasma Spectrometry (LA-ICPMS) and X-ray diffraction techniques. The basal part of the section is characterized by presence of quartz and kaolinite as the major crystalline minerals with minor quantity of hematite. The presence of hematite in the basal part of the shale sequence suggests oxidizing diagenetic environment of deposition. The second geochemically specific interval (upper part) is characterised by quartz and kaolinite as major crystalline minerals with traces of halloysite and grossite. The ternary plot of these major elements indicates the majority of shale samples examined are variably enriched with SiO₂ relative to Al₂O₃ and CaO. The positive correlations of K₂O, TiO₂, and Na₂O, with Al₂O₃ indicate that these elements are associated entirely with detrital phases. Some trace elements such as Cr, Ni, and V are positively correlated with Al₂O₃ which suggest that these elements may be bound in clay minerals and concentrated during weathering. The K₂O/Al₂O₃ ratio is close to the lower limit of clay mineral range, which suggests that kaolinite is the dominant clay minerals. The Al₂O₃/TiO₂ and low Cr/Ni ratios suggest that felsic components were the main components among the basement complex source rocks. The geochemical indices such as Th/Cr, Cr/Th, Th/Co and Th/Cr ratios suggest that these shales were derived from felsic source rocks. The chemical index of alteration values indicates that these shales have experienced strong chemical weathering at the source area. In addition, the depletion of Na and Ca also illustrates an intense chemical weathering of the source rocks. The mineralogical index of alteration values of the studied shale samples indicates an intense to extreme weathering of mineralogical components. The shale units exhibits different degrees of trace-element enrichment, with the approximate order of enrichment relative to an average shale being Co > Pb > Ni > Zr > Cu > Rb > V > Cr > Ba > V > Sr > U. The inverse correlation between Eh, pH, EC and TDS in outcrop Maastrichtian shale samples suggests well oxygenated environment of deposition. In addition, based on previously established thresholds, V/Cr, Ni/Co, Cu/Zn and U/Th ratios support that these shales were deposited under oxidizing diagenetic environment.

Keywords: mineralogy, geochemistry, paleo-redox conditions, trace element enrichments, shales, Mamu Formation, Anambra basin, Nigeria.